Matrix Metalloproteinases and their natural inhibitors (TIMPs) regulate the dissolution of the extracellular matrix in growth and development, inflammatory and neoplastic diseases. The objective of this project is to investigate how cells orchestrate the episodic local dissolution of extracellular matrix by the orderly expression and function of MMPs and their inhibitors. Initially our studies are aimed at determining which MMPs are involved in the degradation of a single substrate (reconstituted fibrils of type I collagen) by a single cell type. To address this question we are employing a variety of molecular and cell biological approaches. Specifically we are pursuing mouse genetic approches (gene knock-out and replacement) in order to determine the function of various MMPs in extracellular matrix remodeling in health and disease. Skeletal tissues develop either by intramembranous ossification, where bone is formed within a soft connective tissue, or by endochondral ossification. The latter proceeds via cartilage anlagen, which through hypertrophy, mineralization, and partial resorption ultimately provides scaffolding for bone formation. By studying the MT1-MMP deficient mouse, a novel and essential mechanism governing remodeling of unmineralized cartilage anlagen into membranous bone, as well as tendons and ligaments was discovered. Membrane-type 1 matrix metalloproteinase (MT1-MMP)?dependent dissolution of unmineralized cartilages, coupled with apoptosis of nonhypertrophic chondrocytes, mediates remodeling of these cartilages into other tissues. The MT1-MMP deficiency disrupts this process and uncouples apoptotic demise of chondrocytes and cartilage degradation, resulting in the persistence of ?ghost? cartilages with adverse effects on skeletal integrity. Some cells entrapped in these ghost cartilages escape apoptosis, maintain DNA synthesis, and assume phenotypes normally found in the tissues replacing unmineralized cartilages. The coordinated apoptosis and matrix metalloproteinase-directed cartilage dissolution is akin to metamorphosis and may thus represent its evolutionary legacy in mammals.